This invention relates to an electronic circuit for determining whether the duration of each incursion by a variable signal on one side of a selected level occurs within a selected range. In the analysis of data from various processes, it is desirable to divide frequency data into smaller ranges indicative of process characteristics, as for example in measuring surface characteristics. Prior methods for such data analysis have been relatively crude, using standard filtering techniques which lack high sensitivity, or have been relatively complex using full scale spectrum analyzers. For low frequency signals of fractional cycles per second up to several hundred cycles per second, it is not possible to easily construct tunable filters of variable pass band with high rejection.